A magnetic tape storage device writes data accepted from a server into magnetic tape which is set in a tape drive, and transmits data read from the magnetic tape to the server.
When reading/writing data, the magnetic tape storage device causes magnetic tape to travel as far as a target location. Consequently, it takes a considerably long time to read/write data in a random location. Therefore, there is a virtual tape system in which a disk device is provided between the server and the magnetic tape storage device and the disk device is utilized as a cache.
A virtual tape system will now be described with reference to FIG. 14. FIG. 14 is a diagram illustrating a configuration of a virtual tape system. As illustrated in FIG. 14, a virtual tape system 900 includes a library device 910 and a virtual tape device 920. Furthermore, the virtual tape system 900 is connected to a server 901 to be able to communicate with each other.
The library device 910 includes magnetic tapes 911 to 914 that stores data and tape drives 915 and 916 in which each magnetic tape is set. Furthermore, the virtual tape device 920 includes a TVC (Tape Volume Cache) 921 that retains a plurality of logical volume data.
Upon accepting writing of logical volume data “A” from the server 901 in such the virtual tape system 900, the virtual tape device 920 retains the logical volume data “A” on the TVC 921. Upon accepting an unmount order of the logical volume data “A” from the server 901, the virtual tape device 920 retains the logical volume data “A” on some magnetic tape in the library device 910. Furthermore, before the amount of all logical volume data exceeds the capacity of the TVC 921, the virtual tape device 920 deletes old logical volume data from the TVC 921.
Furthermore, upon receiving reading of logical volume data “B” from the server 901, the virtual tape device 920 determines whether the logical volume data “B” exists on the TVC 921. Upon determining that the logical volume data “B” exists on the TVC 921, the virtual tape device 920 transfers the logical volume data “B” from the TVC 921 to the server 901.
On the other hand, if the logical volume data “B” does not exist on the TVC 921, the virtual tape device 920 reads the pertinent logical volume data “B” from magnetic rape in the library device 910 and retains the logical volume data “B” on the TVC 921. And the virtual tape device 920 transfers the logical volume data “B” from the TVC 921 to the server 901. By the way, processing of reading logical volume data deleted from the TVC 921, from magnetic tape and storing the logical volume data on the TVC 921 is called recall.
In the virtual tape system 900, the library device 910 is virtually executed on the virtual tape device 920 in this way. As a result, reading/writing accepted from the server 901 is made fast.    Patent Literature 1: Japanese Laid-open Patent Publication No. 11-31376
In the above-described related technique, however, there is a problem that it is not possible in some cases to read logical volume data in a short time at the time of recall processing.
Specifically, in recall processing in the related virtual tape device, the magnetic tape is traveled as far as a location where logical volume data is stored, in response to a mount order, and the logical volume data is read. In a case where the retained logical volume data is written in the rear of a physical tape, however, travel time of the magnetic tape becomes long and consequently it takes a considerably long time to read at the time of recall.